1. Field of the Invention
The invention relates to a method of laser machining for correcting pattern defects on photomasks and liquid crystal substrates, and a laser machining apparatus using the method.
2. Description of the Related Art
One of prior art laser machining apparatuses for correcting pattern defects on photomasks and liquid crystal substrates is disclosed in Japanese Patent Laid-Open No. 2002-62637. In the method for correcting pattern defects disclosed by this application, a substrate is irradiated with a laser beam while its surface to be machined is directed downward thereby removing defects on the substrate by laser vaporization. In this method disclosed, the substrate surface can be observed without lowering the resolution of an optical system for observing it while the substrate surface is being irradiated with a laser beam. Further, since fine particles produced by the laser vaporization can fall down by gravitation, a highly accurate pattern can be formed, and the amount of fine particles resettling on the substrate can be limited to a minimum.
Also, a laser machining apparatus according to the National Publication of International Patent Application No. 1989-502149 (PCT/US87/03488) has a gas enclosure having a blowoff port and inlet port of gas which are shaped concentrically like a donut. This feature allows the local blowing of gas onto the substrate, switching of gases, and switching of the directions of blowoff and suction of gas. Then, one apparatus can perform both the forming of a pattern film by laser CVD and the removing of a pattern film by laser vaporization.
However, laser correction methods using the conventional laser vaporization method require that laser irradiation instantaneously heat a pattern film to be removed to a high temperature of several thousand degrees Celsius, and this causes a problem that the irradiation can dig holes in the substrate surface with a pattern film adhering thereon even if the substrate is a high-temperature-resistant substrate such as quartz. For example, in the case of a Cr mask for semiconductor processes, even if machining conditions such as laser power and pulse duration are optimized, damages with a depth of approximate 10 nm can be made in the substrate surface by the laser irradiation. For this reason, when this mask is used in the actual exposure process, the damages cause changes in the phase of the light for exposure, which can result in a developed pattern width different from the predetermined width. In order to decrease the depth of damages, laser power for correcting patterns may be lowered relative to the optimum condition but this causes a problem that the edge shape of laser beam-machined portions to be degraded in linearity.
In order to resolve these problems, there has been desired a method for etch removing a pattern by irradiation with a laser beam in an atmosphere of an etching gas. However, in the case of photomasks using chromium as a pattern material, which is generally chemically and thermally high resistant, there has been no proposal of etching gas which meets requirements for use in pattern correction operations, such as easy handling, proper reaction speed, and ability to make reaction products having a high vapor pressure.
Further, as disclosed in National Publication of International Patent Application No. 1989-502149 (PCT/US87/03488), in order to effectively provide a local atmosphere of a source gas, it is required that a gas enclosure for locally blowing gas on a substrate window have inlet port and blowoff port provided concentrically and centrally symmetrically around a laser irradiation portion. However, it is very difficult to construct such dual gas flow paths concentrically arranged around the laser irradiation portion in the gas enclosure, within the range of a usual gap distance of only about 1 mm, between the substrate surface and the gas enclosure, the distance depending on the operating range of the objective lens. Thus, this causes problems of high fabricating cost and low construction yield of the gas enclosure. In a high precision laser machining apparatus, the high resolution of the apparatus requires a shorter operating range, so that it is no longer possible to ensure a space for the gas enclosure. This causes a problem that a high precision machining apparatus cannot have such a gas window.
Further, in such a case where concentric blowoff and suction ports of gas are provided in the gas enclosure, in order to increase the concentration of a source gas at the machined portion and to reduce the adherence of fine particles produced at the machined portion to the substrate, the source gas may blow off from a small-diameter nozzle so that the source gas at the laser irradiation portion can be given high flow velocity. In this case, the gas shield effect of the blowoff port is broken, resulting in problems such as the leakage of the source gas into the surroundings, the mixing of air, and the like.